A high-voltage rectifier unit is a device intended for conversion of a low-voltage alternating current into a high-voltage direct current. Devices designed for converting an alternating-current voltage into a direct-current one are well known and widely used in various fields of technology to supply electric and electronic devices, motors and the like. A transformer and a rectifier means are their basic elements. Depending upon the application and the parameters of high-voltage rectifier units they employ transformers of various power and type, while vacuum tubes and semiconductor devices with a unidirectional conduction are used as the rectifier means. Certain high-voltage rectifier units employ a plurality of rectifier means connected in series or in parallel and forming an integral construction element which is a rectifier device. The high-voltage rectifier unit operates as follows.
The alternating-current voltage to be converted into a direct-current voltage of a desired value is applied to the primary winding of the transformer whereas the secondary winding of the transformer supplies an alternating-current voltage to the rectifier means of rectifier devices which are interconnected and connected to the secondary winding of the transformer.
A load is connected in series with the output of the rectifier devices so that the current flowing through it is unidirectional as in the rectifier devices, i.e. is a direct current.
The value of the rectified voltage depends upon the transformer type and the voltage applied. The value of the rectified voltage can be adjusted by changing the voltage applied to the primary winding of the transformer. The unavoidable ripple of the rectified voltage can be reduced or eliminated by the use of special electric filters as well as by selecting an appropriate rectification circuit.
Most problems arise in construction of high and superhigh voltage rectifier units. In these cases, special measures are to be taken to provide the required dielectric strength of the rectifier unit by increasing the insulating gaps, the overall dimensions of the rectifier unit thus growing extraordinarily. This makes impossible to use them in an open construction for certain applications. It is especially difficult to construct high-voltage rectifier units for tens and hundreds of kilovolts. In this event the principal elements of the high-voltage rectifier unit--the transformer and the rectifier devices--are enclosed in a hermetically sealed vessel filled with an electrically insulating fluid, for example, transformer oil or insulating gas. In order to increase the dielectric strength of the gas it is usually used under pressure of several atmospheres. The high rectified voltage is derived from the rectifier unit through the vessel walls to a load located outside the vessel, using a high-voltage lead.
Known in the prior art is a high-voltage rectifier unit (cf. USSR Author's Certificate No. 546 128 dated 1976) that comprises a three-phase transformer, rectifier devices mounted on supporting insulators, a high-voltage electrode, and a high-voltage lead all enclosed in a vessel, filled with an electrically insulating fluid.
However, the construction features of the high-voltage rectifier unit mentioned hereinabove enlarge its overall dimensions and increase its weight since the axial dimension of the high-voltage rectifier unit is increased due to three insulating gaps used to provide the specific conditions of operation for this rectification circuit. The normal operation of the rectifier unit requires high-voltage insulation between circuit elements which have different potentials. In the high-voltage rectifier unit under consideration using the three-phase bridge circuit, the beginnings of the transformer secondary windings connected to the rectifier devices should be insulated from grounded parts of the transformer magnetic core, and this is provided by respective insulating gaps. The ends of the secondary windings interconnected in a common point should be insulated from the high-voltage electrode located above the secondary windings. This is provided by one more insulating gap. The high-voltage electrode should be also insulated, in turn, from the grounded parts of the transformer magnetic core. This is provided by a respective insulating gap. The three high-voltage insulating gaps not only increase the outline dimensions of the vessel and its weight, but also the length of the magnetic core, i.e. the amount of electrical steel used. This leads, in turn, to an increase in the electric loss and to a decrease in technical and economical characteristics of the high-voltage rectifier unit.
Furthermore, the increased number of high-voltage gaps reduces the service reliability of the high-voltage rectifier unit owing to an increased probability of electric breakdowns therein.
In the high-voltage rectifier unit mentioned above, rectifier devices are used that operate at a reverse voltage applied thereto that exceeds the high rectified voltage. At the present time, rectifier devices for such high voltages are not commercially available. Hence, the implementation of such a high-voltage rectifier unit requires a special construction of rectifier device made up of a plurality of standard rectifier means connected in series. This also reduces the service reliability of the high-voltage rectifier unit owing to a non-uniform voltage distribution among rectifier means of the rectifier device. The latter occurs mainly owing to capacitive currents flowing between rectifier means of the rectifier device and structure elements of the high-voltage rectifier.
Transient conditions caused, for example, by a short-circuit in the load or a breakdown of the insulating gaps produce particularly unfavourable voltage distribution among the rectifier means in such a rectifier device. The voltage across certain rectifier means can reach an intolreable value at which the rectifier means can fail. A similar situation can take place also in the secondary winding of the transformer made as a single cylindrical coil with a very large number of turns. Here transients can produce overvoltages between adjacent turns, that can damage the turn-to-turn insulation. As a result the replacement of the entire secondary winding coil may become necessary. The reliability of the high-voltage rectifier unit is also impaired by the use of supporting insulators used for mounting the secondary windings and rectifier devices. The insulator length is known to increase not directly as the voltage increases since this is accompanied by a reduction in the breakdown voltage. At high voltages the probability of breakdown increases for long supporting insulators due to local field intensities on the insulator, arising from non-uniform electric-charge distribution over its surface.
Also known to the prior art is a high-voltage rectifier unit (pre-print No. 74-11, 1974, IJaF AN SSSR) comprising, arranged in a vessel filled with an electrically insulating fluid, a transformer having a sectionalized secondary winding each section of which is provided with a metallic shield and includes two coils interconnected in series by the shield, each being provided with an inner and an outer lead, a high-voltage electrode, a high-voltage lead, rectifier devices installed on supporting insulators, each section of the secondary winding of the transformer being connected with one of the rectifier devices by means of one of the outer leads and forming a rectification stage. The direct-current voltage rectification stages are connected in series so that the voltages of the rectification stages are added, whereby at the output of the last stage connected with the high-voltage electrode the required high voltage is developed. This voltage is applied to a load installed outside the vessel by means of the high-voltage lead made as a high-voltage through insulator mounted on the vessel.
The high-voltage rectifier unit described hereinabove has only one high-voltage gap, i.e. the gap between the high-voltage electrode and the grounded parts of the transformer magnetic core and those of the vessel. Since the secondary winding is sectionalized, the insulating gaps between the beginning of first stage coil windings and the earthed parts of the transformer magnetic core and of the vessel as well as those between the ends of the last stage coil windings and the high-voltage electrode are made minimum and designed for the voltage of one stage only. As a result, the axial dimension of this high-voltage rectifier unit is reduced, and the length of the transformer magnetic core is shortened as compared to other prior art rectifier units. Thus, the overall dimensions of the high-voltage rectifier unit and its weight are reduced. At the same time the service reliability of the high-voltage rectifier unit is improved due to the reduction in the number of high-voltage insulating gaps designed for the full rectified voltage. The shortened length of the transformer magnetic core enables the amount of metal used for the transformer manufacturing to be reduced whereby, its electrical loss is also reduced. This improves the technical and economical characteristics of the unit. The sectionalized secondary winding opens the way for the use of commercially available rectifier devices, i.e. rectifier devices designed for the voltage rectified in each stage, which is lower than the high output voltage and depends upon the number of stages. The number of stages can be selected depending upon the required high voltage and the allowable voltage of each rectifier device. Therefore the number of rectifier means connected in series in a rectifier device of the stage is reduced to a minimum, thus improving the service reliability of the high-voltage rectifier unit.
The number of turns in coils of each section is also reduced as a result of the use of the sectionalized secondary winding, and, hence, the probability of section coil failure during transients is decreased, i.e. the service reliability of the high-voltage rectifier unit is improved as well.
At the same time in case of a damage of the secondary winding, only damaged sections should be replaced and not the entire secondary winding.
In the high-voltage rectifier unit mentioned herein-above, individual supporting insulators designed for the voltage of one stage are installed instead of the supporting insulators designed for the full high voltage. It is known that the sectionalized insulator with a forced uniform potential distribution along its height possesses a higher dielectric strength. This results in a higher service reliability of the high-voltage rectifier.
However, this high-voltage rectifier is a rather complicated one, since it includes a great number of elements interconnecting separate parts, as well as a great number of rectifier means. Actually, three insulation plates are used in each stage for installation of the rectifier devices in this high-voltage rectifier, and these plates are, in turn, installed on three supporting insulators. Each section of the secondary winding is secured to respective insulation plates.
Thus, the whole assembly of the sections and the rectifier devices forms a stack including a great number of insulating elements. In such a stack the stiff fitting of sections to the insulation plates cannot provide a coaxial arrangement of the secondary winding with respect to the primary winding of the transformer due to a great number of connecting elements, which can lead local electric-field non-uniformities in the gap between the primary winding and the secondary winding and, as a result, to a reduced dielectric strength of this insulating gap and, hence, to a decreased reliability.
Furthermore, the service reliability of such a high-voltage rectifier unit is also impaired as the number of insulating elements increases.
The above mentioned construction of the high-voltage rectifier unit requires complete disassembling of rectification stages to replace a damaged rectifier means.
Moreover, the stiff fitting of the sections to the insulation plates can cause a damage to the coils, which also impairs the reliability of the high-voltage rectifier unit.
Thus, the prior art high-voltage rectifier unit is of a complicated design and has a relatively low service reliability. It is an object of the present invention to improve the reliability and to simplify the design of the high-voltage rectifier unit.